The present invention relates to a controller for regulating the input impedance of pulse width modulation converters (PWM converters), and) more particularly, to a controller for regulating the input impedance of pulse width modulation converters for active power factor correction.
PWM converters are widely used for DC-DC, DC-AC, AC-DC and AC-AC conversion. In some cases the purpose of the PWM converter is to shape the input current waveform as seen at the input of the converter. For example, in active power factor correction (APFC), the function of the converter is to ensure that the AC current seen by the source is in phase with the source voltage and has minimum higher-order harmonics. The present application uses the term "pulse width modulation converter" and the equivalent term "PWM converter" to denote any circuit which actively switches reactive components in a network in such a way as to control the impedance of the network by varying the timing of the active switching. An important class of PWM converters is based on what is referred to as the "boost topology." A typical well-known embodiment of a PWM converter in the boost topology is used as an APFC circuit and is shown in FIG. 1. In this "boost converter", the input voltage is rectified by a full-wave rectifier D.sub.1 and fed to a boost stage with an input inductor L.sub.in, a duty switch S.sub.1, a high frequency rectifier D.sub.2 an output filter capacitor C.sub.o and a load R.sub.L. Duty switch S.sub.1 is opened and closed by a high frequency control signal in such a way as to force the input current (i.sub.in) to follow the shape of the rectified input voltage (v.sub.inR). Because of this, the input terminal will look resistive rather than reactive. That is, the power factor (PF) will be equal to unity. By varying the timing of the active switching in other ways, the input impedance of the boost converter can also be made to look reactive, either capacitive or inductive, or as a combination of reactive and resistive loads. In all such applications, the controlling of the timing of the active switching (duty switch S.sub.1 in FIG. 1) is crucial to attaining the desired results.
A control circuit is required for a PWM converter, in order to operate duty switch S.sub.1 properly to achieve the desired shaping of the input current waveform, and the present application uses the term "control circuit" to denote any mechanism which performs or governs the switching for a PWM converter. The present application uses the term "duty cycle" to denote any repetitive sequence of switching characterized by a first time period for a given state followed by a second time period of the opposite state, the sequence taking place over a given time interval. The present application uses the term "duty cycle period" to denote the given time interval of the duty cycle. The present application uses the term "duty switch" to denote any device which controls electricity and which may be characterized by two opposite states. Examples of duty switches include, but are not limited to, transistors, relays, and the like, and is illustrated in a circuit as duty switch S.sub.1 in FIG. 1. A duty cycle is illustrated in FIG. 2. For convenience in reference, the present application uses the terms "opened" and "closed" to denote the two opposite switching states, and also uses the equivalent designations "off" and "on" respectively. In FIG. 2 the total duty cycle period is a period T.sub.S. During duty cycle period T.sub.OFF the duty switch is opened, and during duty cycle period T.sub.ON the duty switch is closed. The control circuit for a PWM converter generates a signal characterized by a duty cycle having a specified ratio of the closed switching time period to the total duty cycle time period (T.sub.ON /T.sub.OFF). The present application uses the term "regulating the duty cycle" to denote the adjustment of this ratio of the closed switching time period to the total duty cycle time period. It is also common in the art to refer to the varying of the duty cycle time periods as a "pulse width modulation," wherein the term "pulse width" refers to the closed or opened switching period, as illustrated in FIG. 2, and wherein the term "modulation" refers to the varying of these periods. In the art there are different regimes for performing pulse width modulation. It is possible to hold the total duty cycle period constant and vary the opened and closed time periods, such that as one increases the other decreases. It is also possible to hold either the opened or the closed time period constant and increase the other, so that the total duty cycle period increases.
Prior art control circuits are complicated, and in particular, they require an analog multiplier and sensing of the input voltage as a reference to the desired shape of input current. This adds to the cost and increases the sensitivity to noise and temperature changes. A multiplier is needed when a boost converter is operated in what is referred to as the "continuous current mode" (CCM). In the continuous current mode, the current through inductor L.sub.in. (FIG. 1) does not drop to zero during a given duty cycle. Furthermore, existing control circuits need to sense the input voltage in order to regulate the current properly. It is well-known in the art that this may introduce further noise as well as to create distortion of the input current.
For what is referred to as the "discontinuous current mode" (DCM), the prior art control circuit is simpler and does not require an analog multiplier. However, as is known in the art, DCM control circuits are less efficient and are unsuitable for high power use.
There is thus a widely recognized need for, and it would be highly advantageous to have, a continuous current mode active power factor correction control circuit which does not require an analog multiplier and does not need to sense the input voltage. It would also be desirable to be able to construct such a control circuit using fewer components of lower cost. These goals are met by the present invention.